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Physics of Future Information Technologies

The ever-decreasing size of the basic components of information processing will give quantum effects an important role in future technologies. Recent developments such as quantum cryptography and the idea of a quantum computer have shown that, rather than being only harmful, these effects can probably be utilized to a great extent.

In communications technologies, optical transmission is setting the trend in the development of the networks. The full harnessing of the huge bandwidth provided by light still requires for replacing the swithing, routing and processing electronics by all-optical components. Research on nonlinear optical materials and light-induced quantum effects will be crusial in the development of future all-optical processing technologies.

We have investigated cold atomic Fermi-gases which can be used for studing important quantum many-body effects such as superconductivity. Cold atomic gases may also serve as a source of atoms in quantum information processing applications. This research is done in collaboration with the University of Innsbruck, Austria, with Nordita, Denmark, and with the Loomis Laboratory, University of Illinois, USA. We have also participated in the University of Jyväskylä -based experimental research on the idea of using superconducting Josephson junctions as the basic processing element of a quantum computer. Related to optical communications technology, we have started a project investigating the possibilities of all-optical swithing and processing using nonlinear materials, combined with novel material structures such as photonic crystals.

All-optical Switching with Non-linear Photonic Crystals

Researchers: Anu Huttunen and Päivi Törmä

Photonic crystals are periodic structures of dielectric media with alternating dielectric constants. The periodicity causes bandgaps for light to appear, i.e., light with a certain wavelenght cannot travel in the crystal. Photonic crystals are a very attractive solution to various problems in telecommunications. The periodicity, and thus the bandgap, can be in either one, two or three dimensions. A widely used example of a one-dimensional photonic crystal is the Bragg grating. Two-dimensional photonic crystals could be used, e.g., as a waveguide for integrated optics (see Fig. 36) and three-dimensional photonic crystals as a microcavity.

We study photonic crystals made of nonlinear materials. The nonlinearity means that the material properties are dependent on the local light intensity. Thus also the bandstructure of the photonic crystal is dependent on the intensity of light in the crystal: One light signal can change the bandstructure for the other light signal or the signal can change the bandstructure for itself. To calculate the bandstructure for nonlinear photonic crystals, we use an iterative Fourier-method. This phenomena could be used for all-optical switching in optical telecommunications. Nowadays the switching of light signals in optical networks is done mainly electronically, which imposes a bottleneck for the data transmission.

  
Figure 36
Figure 36: Simulation of a two-dimensional photonic crystal acting as a waveguide. Sharp bends, which enable the use of integrated optics, are realizable with photonic crystals. Figure is from J. D. Joannopoulos, R. D. Meade, and J. N. Winn, ``Photonic Crystals'', Princeton University Press (1995).

Degenerate Atomic Fermi Gases

Researchers: Mirta Rodriguez, Gheorghe-Sorin Paraoanu*, and Päivi Törmä
*University of Illinois at Urbana-Champaign

The achievement of Bose-Einstein condensation (BEC) in alkali gases has stimulated the trapping and cooling of also the Fermionic isotopes. The regime of quamtum degeneracy has already been reported for 40K and the first signatures of Fermi statistics have been observed.

Atomic gases can be efficiently and accurately manipulated. They are dilute and weakly interacting thus offering the ideal tool for studying fundamental quantum statistical and many-body physics.

The most prominent phenomena for the fermionic samples would be the superfluid BCS transition associated with the appearance of the order parameter macroscopic wave function (Cooper pairs). It is an open question how to observe the BCS-transition because the gap is predicted to be small and no significant changes in the bulk properties of the gas are expected.

We have proposed the use of on-resonant light and have studied the effects of the trapping potential on this method. We are studying also the spatial correlations between the atoms in the trap and exploring macroscopic quantum effects of the superfluid such as vortices.

 
Figure 37
Figure 37: Spatial distribution of the order parameter when there is a single vortex. Numerical solution of the Ginzburg-Landau equation for trapped 6Li.


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